1. Field of the Invention
The present invention relates in general to a method and an apparatus for inspecting a spark plug or ignition plug for defects thereof such as defects of a discharge gap and a porcelain thereof, and more particularly to such method and apparatus which permit the inspection of the spark plug while the spark plug is installed in an engine.
2. Discussion of related Art
Since spark plugs are relatively likely to be defective during installation thereof in an engine, the inspection of the spark plugs is desirably conducted after the installation in the engine. An example of a method of inspecting spark plugs for their discharge gaps while the spark plugs are installed in an engine. According to this inspection method, the secondary voltage of an ignition coil is detected by an exclusive probe while the engine is operated by combustion of an air-fuel mixture, so that the spark plug is inspected for adequacy of the discharge gap based on a time duration of an inductive discharge component of the detected secondary voltage.
This inspection method permits the inspection of the spark plug as installed in the engine. However, this method is a so-called "firing check" in which the engine is operated by combustion of the air-fuel mixture. Further, the method relies on only the time duration of the discharging. Therefore, the conventional inspection method cannot be considered satisfactory. The discharging in the spark plug tends to be influenced by the pressure and the fuel concentration of the air-fuel mixture surrounding the discharge gap of the spark plug. In the firing state of the engine, there exists a relatively strong fluid flow within the cylinder of the engine, which may cause considerable variations in the pressure and fuel concentration around the discharge gap, leading to insufficient accuracy of inspection of the discharge gap.
It is desirable to inspect the spark plug for a defect of the porcelain, as well as for the adequacy of the discharge gap. The conventional inspection method indicated above cannot satisfy this need.